A floating iceberg off the Antarctic Peninsula. Photo courtesy
CU-Boulder National Snow and Ice Data Center.
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University of Colorado at Boulder researchers have used data from a pair of NASA satellites orbiting Earth in tandem to determine that the Antarctic ice sheet, which harbors 90 percent of Earth's ice, has lost significant mass in recent years.

The team used measurements taken with the Gravity Recovery and Climate Experiment, or GRACE, to conclude the Antarctic ice sheet is losing up to 36 cubic miles of ice, or 152 cubic kilometers, annually. By comparison, the city of Los Angeles uses about 1 cubic mile of fresh water annually.

"This is the first study to indicate the total mass balance of the Antarctic ice sheet is in significant decline," said Isabella Velicogna of CU-Boulder's Cooperative Institute for Research in Environmental Sciences, chief author of the new study that appears in the March 2 online issue of Science Express. The study was co-authored by CU-Boulder physics Professor John Wahr of CIRES, a joint campus institute of CU-Boulder and the National Oceanic and Atmospheric Administration.

The estimated ice mass in Antarctica is equivalent to 0.4 millimeters of global sea rise annually, with a margin of error of 0.2 millimeters, according to the study. There are about 25 millimeters in an inch.

The most recent Intergovernmental Panel on Climate Change assessment, completed in 2001, predicted the Antarctic ice sheet would gain mass in the 21st century due to increased precipitation in a warming climate. But the new study signals a reduction in the continent's total ice mass, with the bulk of loss occurring in the West Antarctic ice sheet, said Velicogna.

Researchers used GRACE data to calculate the total ice mass in Antarctica between April 2002 and August 2005 for the study, said Velicogna, who also is affiliated with the NASA's Jet Propulsion Laboratory in Pasadena.

"The overall balance of the Antarctic ice is dependent on regional changes in the interior and those in the coastal areas," said Velicogna. "The changes we are seeing are probably a good indicator of the changing climatic conditions there."

Launched in 2002 by NASA and Germany, the two GRACE satellites whip around Earth 16 times a day at an altitude of 310 miles, sensing subtle variations in Earth's mass and gravitational pull. Separated by 137 miles at all times, the satellites measure changes in Earth's gravity field caused by regional changes in the planet's mass, including such things as ice sheets, oceans and water stored in the soil and in underground aquifers.

A change in gravity due to a pass over a portion of the Antarctic ice sheet, for example, imperceptibly tugs the lead satellite away from the trailing satellite, said Velicogna. A sensitive ranging system allows researchers to measure the distance of the two satellites down to as small as 1 micron -- about 1/50 the width of a human hair -- and to then calculate the ice mass in particular regions of the continent.

"The strength of GRACE is that we were able to assess the entire Antarctic region in one fell swoop to determine whether it was gaining or losing mass," said Wahr. While the CU researchers were able to differentiate between the East Antarctic ice sheet and West Antarctic ice sheet with GRACE, smaller, subtler changes occurring in coastal areas and even on individual glaciers are better measured with instruments like radar and altimeters, he said.

A study spearheaded by CIRES researchers at CU-Boulder and published in September 2004 concluded that glaciers on the Antarctic Peninsula - which juts north from the West Antarctic ice sheet toward South America -- sped up dramatically following the collapse of Larsen B ice shelf in 2002. Ice shelves on the peninsula -- which has warmed by an average of 4.5 degrees Fahrenheit in the past 60 years -- have decreased by more than 5,200 square miles in the past three decades.

As Earth's fifth largest continent, Antarctica is twice as large as Australia and contains 70 percent of Earth's fresh water resources. The ice sheet, which covers about 98 percent of the continent, has an average thickness of about 6,500 feet. Floating ice shelves constitute about 11 percent of the continent.

The melting of the West Antarctic ice sheet alone - which is about eight times smaller in volume than the East Antarctic ice sheet -- would raise global sea levels by more than 20 feet, according to researchers from the British Antarctic Survey.

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